The present invention relates generally to record carriers to be recorded and/or reproduced by optical means, and drive units for driving the record carriers, and more particularly to a disc-shaped record carrier having a characterized record structure or format, and a drive unit for recording and/or reproducing the record carrier. The present invention is suitable, for example, for an optical disc that includes both a read only (referred to as “ROM” hereinafter) area that allows a reproduction of prerecorded data, and a recordable (referred to as “RAM” hereinafter) area that may record new or additional data.
Optical discs, spread as an optical record carrier, include ROM media, such as a CD-ROM and a DVD (Digital Versatile Disc), write-once CD-Rs, other various magneto-optical record carriers, and phase change record carriers, such as a DVD-RAM. Along with the recent developing information industry, demands to optical record carriers have ranged not only the higher density but also efficient arrangements and applications of a wide variety of information. An optical disc that makes available both ROM and RAM areas has been conventionally proposed so as to meet one of such demands For example, the DVD standard stipulates for an adhesion of two disc plates and an adhesion of one disc plate to a dummy disc plate with prerecorded surface (or signal surface) facing up. An optical disc with a ROM area on its one surface and a RAM area on its other surface has been known in the art as disclosed, for example, in Japanese Laid-Open Patent Application No. 10-228673, but there exists a demand to provide one disc surface with both ROM and RAM areas.
Accordingly, the ISO (International Organization for Standardization) standard (e.g., ISO/IEC13963) regulates a partial ROM type magneto-optical disc including a pre-pit formed ROM area, and a data record area formed by address data pits and servo grooves on the same plate. In addition, a certain type, such as a DVD-RAM, forms a ROM area for storing control and other information for the entire disc on its inner (circumference) portion and a RAM area on its outer (circumference) portion. It has also been known in the art to provide both RAM and ROM areas on the same plane in one optical disc as disclosed, for example, in Japanese Patent No. 3,063,641.
For example, it is conceivable to form the ROM and RAM areas along one spiral track and to assign consecutive addresses to it. However, this would change addresses in the RAM area depending upon a length of ROM data and disadvantageously make undefined disc's control area that stores exchange and other information, resulting in the unstable recording and reproducing actions. In addition, without the prefixed capacity of the ROM area, the efficient arrangement of the data record area would be unavailable and it would be difficult to efficiently record data in a finite record area in a disc. Moreover, a drastic change in conventional drive's structure would undesirably make the drive larger, more complicated, and more expensive.
Thus, the way of arranging, addressing, recording and/or reproducing ROM and RAM areas would be critical for an optical disc having these ROM and RAM areas on the same plane.